The invention relates generally to blood pumps of the type in which a rotor, having impeller blades, is supported by magnetic bearings within a stator, and more particularly to preventing pump failure or hemolysis if the rotor should come into contact with the stator.
The number of donor hearts needed for persons having advanced heart failure has not decreased and consequently the need for a long-term alternative to heart transplantation remains. A fully implantable blood pump and system which is smaller than presently available systems and has the high reliability required for long term implantation would be a solution. To address this need, a variety of continuous flow blood pumps have recently been developed to address these requirements.
Continuous flow pumps generally have a rotor portion that has impeller blades for the pumping of blood and a surrounding stator which has features that mechanically support and turn the rotor to generate flow via the impeller blades. Some of these pumps have a mechanical bearing to support the rotor while others support the rotor in part or whole using a magnetic suspension system. Pumps which have mechanical bearings have the potential to cause hemolysis (blood damage) due to mechanical trauma or to heat generation, both of which are induced by the contact regions of the mechanical bearing. Some pumps employ a hydrodynamic bearing with the blood as the liquid portion of the bearing. Although much work has been done to determine the time duration and shear stress level at which hemolysis occurs, this type of rotor support has unknown long-term effects on blood. Pumps having magnetic suspension have the advantage of rotor-stator interaction that doesn""t require contact or the extremely close tolerance between the rotor and stator of a hydrodynamic bearing, both of which induce mechanical trauma. However, one limitation of magnetic suspension is the control of the rotor during power failure or excessive mechanical shock to the blood pump. In these instances, the rotor may crash into the stator and cause surface damage to both components. In addition, most blood pump rotors use impeller blades for the pumping of blood. The blades are typically thin and consequently provide a small surface area for contact between the rotor and stator. The small surface area provided by the blade tips increases the likelihood of local surface damage. A larger surface area is better than a smaller one since the transfer of energy between the impacting components is spread out to a greater extent and consequently the surface damage will be less. Regardless of the size of the contact area, the touch down event can cause hemolysis and/or damage (scratch or gouge) the contacting surfaces of the rotor and/or stator which may subsequently cause thrombosis (blood clot) by providing a crack or crevice for the blood to begin depositing cells or other blood products.
One example of a blood pump in which the rotor is entirely supported by magnetic bearings is described in U.S. Pat. No. 4,688,998. When the operation of this blood pump is halted due to a power failure, the rotor shifts toward the inlet of the blood pump to block the backflow of blood through the blood pump. A portion of the rotor, referred to as the valve body, will contact a region of the stator, referred to as the valve seat, during power failure. No provision is made to have the rotor and stator portions designed to tolerate repeated impacts without damage to the blood contacting surfaces. This embodiment is again described in U.S. Pat. No. 4,944,748, which discloses additional embodiments of blood pumps that have magnetically suspended rotors. These embodiments likewise have no unique features for the tolerance of contact between the rotor and stator.
Another type of magnetically suspended blood pump is described in U.S. Pat. No. 6,050,975. This blood pump is designed to have a textured blood-contacting surface that promotes the growth of a biologic lining from the passing blood. Although this technique has been shown to produce beneficial results from the standpoint of preventing unstable clot formation, contact between the rotor and stator due to a power failure would potentially break loose tissue from the textured surface. Consequently, this pump cannot tolerate rotor-stator contact without causing serious harm to the patient.
Generally, touch down events may be grouped into two categories: touch down due to power failure; or touch down due to mechanical shock. If a blood pump power failure occurs, the rotor may, in certain designs, be slammed into the stator by the un-powered and consequently unbalanced magnetic bearings. For a well designed blood pump, the chance of a power failure is highly unlikely. However, for the safety of the patient, the blood pump must be designed to survive and correctly function after such a catastrophic event.
In contrast to touch down caused by a power failure, touch down due to mechanical shock is more difficult to account for, given the difficulty to predict the shock loading a patient may see if they are involved in an accident. One important consideration for determining the required magnetic suspension strength is the capability of the magnetic suspension to withstand the mechanical shock loading from everyday activity. In addition, there are considerations regarding the natural frequency of the magnetic suspension as a function of impeller rotational speed. Both of these issues tend to encourage a stiff magnetic bearing for rotor suspension. A stiffer suspension will enable larger shock loads to be tolerated without touch down occurring. Unfortunately, a stiffer suspension can also result in higher touch down loading if a power failure occurs.
It should be noted that touch down resulting from a mechanical shock will normally occur for a brief time period, typically only for an instant. In contrast, touch down resulting from a power failure can potentially bring the rotor to a complete stop, since the magnets will hold the rotor in place while the rotational energy is dissipated.
Accordingly, there is a need for a blood pump designed to eliminate surface damage that can occur if the rotor should come into contact with the stator.
A blood pump is provided according to the invention wherein portions of the rotor and/or stator are designed to eliminate surface damage if the rotor and stator should come into contact with each other. This can be accomplished generally by specially designing the portions of the rotor and stator which are likely to come into contact as a result of power failure or mechanical shock. In particular, likely touch down contact surfaces of the rotor and/or stator can be made from materials having properties such that generally even the highest touch down forces would not cause any surface damage. Alternatively, the geometry of the likely touch down contact surfaces of the rotor and/or stator can be designed such that the touch down forces are spread across the largest possible surface area to reduce the contact stresses. Moreover, a combination of the choice of materials and the design of the geometry of the likely touch down contact surfaces can be employed to achieve the desired results of eliminating surface damage in the event of rotor touch down against the stator.
Other details, objects, and advantages of the invention will become apparent from the following detailed description and the accompanying figures of certain embodiments thereof.